Some radiation sources exhibit wavelength drift over time in excess of that tolerable for many applications. For many applications, the wavelength stability is required to be within 0.05 nm from a desired wavelength. This drift becomes increasingly important as the lifetimes over which these radiation sources are to be deployed increases. Factors such as temperature, age, operating power level, etc., all affect the output wavelength. By monitoring at least one of the direction of the wavelength change, the degree of the change and the percentage of the light being radiated at the different wavelengths, any or all factors which may be causing this change can be modified in accordance with the monitored signal via a feedback loop to stabilize the wavelength of the radiation source. Preferably, both the power and the wavelength are monitored. The wavelength may be controlled by altering a temperature of the light source, e.g., by a thermally cooled unit, and the power may be controlled in accordance with the injection current. Since the power and the wavelength are interdependent, i.e., a change in one parameter affects the other, the best stability is achieved when both are controlled.
Such monitoring and stabilizing systems typically involve using a unit which is external to the radiation source itself. Such external units include crystal gratings, fiber gratings, spectrometers, and Fabry-Perot etalons, both straight and inclined. The grating systems include relatively large control units external to the radiation source. While etalon-based systems offer a more compact solution, so far these etalons are still separate units which may become improperly aligned, either with photodetectors or with optical elements required to direct and control the light onto the photodetectors.
While the parent application addresses the above concerns, it would still be advantageous to provide a wavelength locker which is tunable and/or operates over a plurality of wavelengths. Currently, solutions to this desire for tunability include rotating the etalon to match the wavelength operation to one of the lockable wavelengths and using a waveguide grating to separate the wavelengths output by the source. However, these are not robust over a variety of operating conditions and cannot be integrated on a wafer level. Other configurations have used a stepped etalon, but with no integration thereof.